One interesting historical tidbit is about the name itself. For centuries western European mass murderers (I mean kings, or emperors) just used the name Roman Empire to designate their fancy that they rightfully ruled the entire world. We usually distinguish a Western Roman Empire from the Eastern Roman Empire that existed at Constantinople [Istanbul] from the time of Saint Constantine (aka Constantine the Great), but they just called it the Roman Empire.

Constantine's successors made the Empire a Christian theocracy. When the Popes and another mass murderer, Charlemagne, revived the empire in the west of Europe it was Roman Catholic, the Popes having made up some stories about their theological supremacy since the beginning of the Dark Ages.

But this neo-Roman Empire, with the Pope in charge of theology and the Emperor in charge of killing anyone who disagreed, only got the tag "holy" during the reign of Frederick I. He used it in a letter sent in 1157 to his vassals asking them to follow him over the Alps to massacre the Lombard (northern Italian) cities that thumbed their noses at this pretensions to being their ruler.

Frederick did not consistently use the term "Holy Roman Empire." His successors used it on occasion for generations, until by the time of Charles IV it was used consistently and was adopted by most of Europe, even by those resisted the idea that it granted any right to boss around distant kingdoms like France, England, and Spain.

No constitutional changes in the Empire came about with the name change. It may have been a ploy to give the Emperors a leg up in their struggle with the Popes. The Popes claimed to be supreme by their direct connection to God, and the right to recognize or not recognize the Emperor and the Christian kings of Europe. The Emperors believed or at least proclaimed they received the crown directly from God, so that the Pope was in effect their chaplain, a guy they could hire, fire, and disagree with within their royal powers.

Adolf Hitler was a Roman Catholic, put in power partly by a Pope who wanted Hitler to slaughter the atheists (socialists and communists included) of the world. When Hitler was defeated (mostly by the Communists, with some minor help from America and the British Empire) the Pope [Pius XII] tried to pretend he hardly even knew Hitler. Their occasionally vehement disagreements (about the curriculum in schools, for instance) are often trotted out by historians to obscure the overall picture. Hitler was Catholic, and thought the Pope should obey him. The Pope had brought Hitler to power, bent Mussolini to his will, and also was recognized as supreme by General Franco. Their disagreements were among allies, not among enemies.